Self-assembled monolayer and method of making

ABSTRACT

According to the present invention, the previously known functional material having a self-assembled monolayer on a substrate has a plurality of assembly molecules each with an assembly atom with a plurality of bonding sites (four sites when silicon is the assembly molecule) wherein a bonding fraction (or fraction) of fully bonded assembly atoms (the plurality of bonding sites bonded to an oxygen atom) has a maximum when made by liquid solution deposition, for example a maximum of 40% when silicon is the assembly molecule, and maximum surface density of assembly molecules was 5 silanes per square nanometer. Note that bonding fraction and surface population are independent parameters. The method of the present invention is an improvement to the known method for making a siloxane layer on a substrate, wherein instead of a liquid phase solution chemistry, the improvement is a supercritical phase chemistry. The present invention has the advantages of greater fraction of oxygen bonds, greater surface density of assembly molecules and reduced time for reaction of about 5 minutes to about 24 hours.

CROSS REFERENCE TO RELATED INVENTION

[0001] This application is a Continuation-In-Part of application Ser.No. 09/272,762, filed Mar. 19, 1999.

[0002] This invention was made with Government support under ContractDE-AC0676RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention is a self-assembled monolayer and method ofmaking.

BACKGROUND OF THE INVENTION

[0004] Since their unveiling in 1992, mesoporous ceramics have inspiredsubstantial interest, especially by adding self-assembling monolayercompounds to the surface(s) of the mesopores. By varying the terminalgroup of the self-assembling monolayer, various chemicallyfunctionalized materials have been prepared. A mesoporous material isdefined as a material, usually catalytic material, having pores with adiameter or width range of 2 nanometers to 0.05 micrometers.

[0005] Exemplary of use of self-assembling monolayer(s) on a mesoporousmaterial is the International Application Publication WO 98/34723(E-1479 CIP PCT). The self-assembling monolayer(s) is made up of aplurality of assembly molecules each having an attaching group. Forattaching to silica, the attaching group may include a silicon atom withas many as four attachment sites, for example; siloxanes, silazanes, andchlorosilanes. Alternative attaching groups include metal phosphate,hydroxamic acid, carboxylate, thiol, amine and combinations thereof forattaching to a metal oxide; thiol, amine, and combinations thereof forattaching to a metal; and chlorosilane for attaching to a polymer. Acarbon chain spacer or linker extends from the attaching group and has afunctional group attached to the end opposite the attaching group.

[0006] Methods of attaching and constructing the self-assemblingmonolayer on a mesoporous material involve solution deposition chemistryin the presence of water. More specifically, as reported by Feng, X.;Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science,1997, 276, 923-926 (Feng et al, 1997); and Liu, J.; Feng, X.; Fryxell,G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165(Liu et al., 1998), a synthetic protocol to prepare monolayers of MPTMS(mercaptopropyl trimethoxysilane) within the pores of MCM-41 involved a1-hour hydration step, followed by a 6-hour silanation step in refluxingtoluene. At this stage, the silane coverage is limited to approximately3.6-4.0 silane molecules/nm² (this surface density is not enhanced byeither extending the reaction time or increasing silane concentration).Following the silanation with a 2-3 hour azeotropic distillation drivesthe equilibria through the removal of reaction by-products, andincreases this surface density to 5.0-5.2 silanes/nm². This surfacedensity is representative of typical silane-based monolayers. Themonolayer coated mesoporous product is then isolated by filtration,washed extensively and then dried for several days. In summary, theoverall procedure takes about 10 hours of laboratory prep time and 1-10days of drying time. The time is driven by the kinetics of getting theself-assembling molecules into the mesopores and getting the water andany other solvent out of the mesopores.

[0007] The product obtained exhibits a maximum of 40% of the monolayersilicon atoms fully crosslinked for maximizing monlayer stability.Ideally, 100% of the silicon atoms would be fully crosslinked. Fullcrosslinking is having three of the four bonding sites linked to anothersilicon atom via an oxygen atom, with the fourth linked to thefunctional group terminated hydrocarbon chain. However, the presence of“dangling” hydroxyl groups (OH—) cannot be avoided in the solutionmethod and it is the presence of the “dangling” hydroxyl groups thatinterferes with the crosslinking of the monolayer, thus placing apractical upper limit on the number of silicon atoms that are fullycrosslinked of 40%.

[0008] Thermal “curing” of silane monolayers, wherein typical thermalcuring (ca. 150° C.), of a silane monolayer creates a terminal tointernal silane ratio of 1:2 corresponding to about 60% to 65% ofattaching molecules (silicon) fully crosslinked.

[0009] Hence, there remains a need for a mesoporous material havingself-assembling monolayer thereon with a greater fraction of theassembly atoms fully crosslinked. There is also a need for greatersurface density of silicon atoms, which may also be expressed as agreater surface density of monolayer coverage. Finally, there is a needfor a method of making these materials that is less time consuming.

[0010] The main difficulty in functionalizing microporous materials maybe attributed to diffusion of the organic molecules intoto the smallpore channels. In the last few years, both post-silanization and in-situdeposition have been successfully applied to mesoporous materials, inwhich the pore diameter is usually larger than 2 nm. The mesoporousmaterials (usually synthesized using surfactant micelles as templates)have very uniform pore sizes. Because of their high surface area and theopen pore channels; functionalized mesoporous materials have beeninvestigated for many adsorption and catalysis applications. However dueto the large pore size and the amorphous nature of the materials, thesematerials are not likely to find application as size selectivecatalysts.

[0011] A zeolite is any one of a family of hydrous aluminum silicateminerals, whose molecules enclose cations of sodium, potassium, calcium,strontium, or barium, or a corresponding synthetic compound, usedchiefly as molecular filters and ion-exchange agents. Compared to themesoporous materials, the diffusion of organic molecules in zeolites isseverely hindered by the small pore size. Deposition of silanes on theexterior surface is therefore greatly favored over silanation ofinternal surfaces. Heretofore, it had been believed that introducingorganic functional groups to the internal pore surfaces of commercialzeolites to produce size selective microporous catalysts could not beachieved due to the size of the pores.

SUMMARY OF THE INVENTION

[0012] According to the present invention, the previously knownfunctional material having a self-assembled monolayer on a substrate hasa plurality of assembly molecules each with an assembly atom with aplurality of bonding sites wherein a bonding fraction (or fraction) offully bonded assembly atoms (fully crosslinked assembly atoms) with theplurality of bonding sites (the plurality of bonding sites bonded to anoxygen atom) exceeds a maximum compared to solution deposition, andmaximum surface density of assembly molecules greater than for solutiondeposition. For example, with the assembly atom silicon, having 4bonding sites, the bonding fraction maximum for solution deposition was40% as deposited or about 60% to 65% (a terminal to internal silaneratio of about 1:2) after thermal curing, and maximum surface density ofsilane molecules was 5.2 silanes per square nanometer. Note thatcrosslinking fraction and surface density are separate parameters.

[0013] The method of the present invention is an improvement to theknown method for making a self-assembled monolayer on a substrate,wherein instead of a liquid phase solution chemistry, the improvement isa supercritical phase chemistry.

[0014] The present invention has the advantages of greater fraction ofbridging oxygen bonds, and greater surface density of assembly moleculesresulting in a lower defect coating that enhances thermal and chemicalstability or resistance. Further, hydrolysis and deposition is completewithin 5 minutes, a surprising rate enhancement of more than two ordersof magnitude. Not only are the hydrolysis and deposition considerablyaccelerated relative to standard solution methods, but the final dryingphase has been completely eliminated by the use of a supercritical fluidas the reaction medium. The product emerges from the reaction chamberdry and ready to use. This represents considerable timesavings.

[0015] Water is a necessary reactant in the hydrolysis and condensationchemistry of alkylsilanes to form self-assembled monolayers onto ceramicoxide surfaces. It must be present in appropriate (stoichiometric)amounts; too little will result in incomplete deposition andcrosslinking and too much will result in bulk solution phase polymerformation. Experience has shown that approximately 10¹³ water moleculesper square meter of available surface area is optimum. This amounts toapproximately 2 water molecules for each silane to be anchored.

[0016] It is also important that this water be intimately associatedwith the surface and not free in solution. By having the water in closeproximity to the ceramic oxide surface, the silanehydrolysis/condensation chemistry can only take place on the surface;thereby favoring the desired monolayer deposition and avoiding solutionphase polymerization (which leads to bulk amorphous polymer and blockedpores). This association is necessary to obtain any thin filmmorphology, and is critical to obtain clean monolayer formation.

[0017] In addition, the water associated with the ceramic oxide surfacemust be evenly spread out across the surface. This causes the hydrolysischemistry to be uniformly spread out across the ceramic oxide surface,which reduces monolayer defect formation, while at the same timeminimizing bulk polymerization.

[0018] By adding the water first, and allowing it to fully equilibratewith the ceramic oxide surface, Applicants are able to exploit thenatural affinity that these ceramic oxides have for water and are thusable to insure that these important conditions are met.

[0019] Adding water separately to a solution of silanes will result inbulk solution phase polymerization competing with any possible monolayerdeposition. This is counter-productive since it significantly depletesthe amount of silane available to form the monolayer and in the case ofa mesoporous substrate, the bulk amorphous polymer will plug and blockthe pore channels, reducing the available surface area and restrictinginterfacial access, thus eliminating the most desirable features of sucha material.

[0020] In the liquid solution deposition of the prior art, a wastestreamis produced as a mixture of water, methanol, toluene and small amountsof mercaptan that failed to be deposited. It is impractical to separatethis mixture, and therefore the mixture is usually disposed of ashazardous waste. According to the present invention using asupercritical fluid for solution deposition, the only by-product of thereaction (hydrolysis) is an alcohol (e.g. methanol), which is easilyseparated from the supercritical fluid (which can be recovered forrecycling). In fact, the alcohol is of sufficient purity to represent apotential feedstock that can be sold or recycled.

[0021] A further advantage of using a supercritical fluid as thereaction medium is the elimination of flammable solvents, and performingthe reaction under completely non-flammable conditions, which can be asignificant concern upon scale-up.

[0022] The subject matter of the present invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. However, both the organization and method of operation,together with further advantages and objects thereof, may best beunderstood by reference to the following description taken in connectionwith accompanying drawings wherein like reference characters refer tolike elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a graph of percent composition versus time for thehydroxylated silanol.

[0024]FIG. 2a is an NMR spectrum of a 5-minute sample made according tothe present invention.

[0025]FIG. 2b is an NMR spectrum of a 24-hour sample made according tothe present invention.

[0026]FIG. 2c is a peak ratio (percent) versus time for various samples.

[0027]FIG. 3 is an NMR spectrum of a 5-minute sample re-annealed for 30minutes.

[0028]FIG. 4 is a graph of sulfur concentration versus pH for samplesmade according to the present invention and compared to samples made bysolution deposition exposed to corrosive solutions.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0029] According to the present invention, the previously knownfunctional material having a self-assembled monolayer on a substrate,the self-assembled monolayer having a plurality of assembly moleculeseach with an assembly atom with a plurality of bonding sites wherein aportion of the assembly atoms have the plurality of bonding sites bondedto an oxygen atom is improved so that a greater portion of the assemblyatoms have the plurality of bonding sites bonded to an oxygen atom.

[0030] In addition, the known functional material having aself-assembled monolayer on a substrate having a surface density byliquid solution deposition of the self-assembled molecules is improvedso that the surface density is greater.

[0031] The method of the present invention is an improvement to theknown method for making a self-assembled monolayer on a substrate, theself-assembled monolayer having a plurality of assembly molecules eachwith an assembly atom with a plurality of bonding sites, the methodhaving the step of bonding a plurality of oxygen atoms to a fraction ofthe plurality of bonding sites; wherein the improvement is the bondingdone by preparing the self-assembled monolayer in a supercritical fluid.

[0032] The supercritical fluid may be polar or non-polar. Compoundsuseful in the supercritical phase according to the present inventionincluded carbon dioxide, and possibly freons, nitrogen, noble gases,alkanes, alkenes, alkynes, and combinations thereof.

[0033] The thermal curing of the self-assembled monolayer may be duringor after forming of the self-assembled monolayer. In other words, aself-assembled monolayer that has been prepared in a supercritical fluidexhibiting a portion of assembly atoms fully crosslinked to oxygen atomsand a maximum surface density of assembly atoms may be exposed to asupercritical fluid for a time that is effective in convertinginterfering hydroxyl groups to bridging oxygen bonds, thereby increasingthe number of fully crosslinked silicon atoms to a greater portion.Alternatively, a self-assembled monolayer prepared by any other method,for example liquid solution chemistry, may be treated by adding assemblymolecules to the gaps of the alternatively prepared monolayer. Theassembly molecules are added in a supercritical fluid containingadditional assembly molecules. The greater portion is at least about75%, preferably greater than or equal to 80%.

[0034] The assembly atoms are selected to be compatible with thesubstrate. For attaching to silica, the assembly atom may include asilicon atom with four attachment sites, for example siloxane, silazane,and chlorosilane. Alternative assembly molecules include metalphosphate, hydroxamic acid, carboxylate, thiol, amine and combinationsthereof for attaching to a metal oxide; thiol, amine, and combinationsthereof for attaching to a metal; and chlorosilane for attaching to apolymer. A carbon chain spacer or linker may extend from the assemblyatom and has a functional group attached to the end opposite theassembly atom.

[0035] For silicon atoms having four bonding sites, the portion of fullycrosslinked bonding sites by supercritical fluid solution exposureincluding supercritical fluid solution deposition is greater than orequal to about 40% as deposited. Additional exposure time increases thefully crosslinked fraction to at least about 55%. Table 1 shows theamount of time for the percent of fully crosslinked siloxanes forsupercritical fluid processing at 7500 psi and 150° C. The times andpercent of full crosslinking are pressure and temperature dependent.TABLE 1 Supercritical CO₂ Exposure Time for Percent of Fully CrosslinkedSiloxanes Time (hours) % of Fully Crosslinked Siloxanes ≦{fraction(1/12)} >40  4 55 24 75

[0036] In addition, supercritical fluid solution deposition results ingreater surface density of the assembly molecules. For siloxane, thesurface density is greater than 5.2 siloxane molecules per squarenanometer, and has been demonstrated up to 6.5 siloxane molecules persquare nanometer.

[0037] Alternatively, or additionally, the surface deposition of theself-assembling monolayer(s) may be done in a manner of placing one ormore self-assembling monolayer precursor(s), including but not limitedto alkoxysilane, silazane, chlorosilane, and combinations thereof,together with mesoporous material that may be ceramic, for example metaloxide, including but not limited to silica, alumina, titania, andcombinations thereof, in a vessel that is subsequently filled with asupercritical fluid, including but not limited to carbon dioxide (CO₂),ethane (C₂H₆), ammonia (NH₃), and combinations thereof, to obtain theself-assembling monolayer(s) on the mesoporous or zeolite material. Byusing a supercritical fluid for deposition of the self-assemblingmonolayer, the surface density of silanes may be greater than 5.2silanes per square nanometer. The surface density is controlled by theamount of assembly molecule (e.g. silane) used for a given surface areaof mesoporous material. Moreover, deposition is complete in about 5minutes and no subsequent drying is needed. With a 5-minute deposition,the percent of fully crosslinked silion atoms is about 40%. Additionalsupercritical fluid exposure time increases the percentage of fullycrosslinked silicon atoms (see Table 1 above).

[0038] Further, the placing of the calcined mesoporous material mayinclude mixing a sol-gel solution and surfactant for producing amesoporous green body; removing the surfactant with the supercriticalfluid and making a dry green body; and calcining said dry green bodyinto the claimed mesoporous material. In this manner, the entire processfrom sol-gel templating through self-assembling monolayer deposition toincreasing the fraction of fully bonded silicon atoms may be done in asingle vessel in a supercritical fluid environment.

EXAMPLE 1

[0039] An experiment was conducted to test the influence ofsupercritical carbon dioxide (SCCO₂) on the hydration of a mesoporoussilica designated MCM-41, obtained by making the MCM-41 according toU.S. Pat. No. 5,264,203 (Mobil Oil Corporation, Fairfax, Va.). Thecalcined substrate (primarily Q4 [non-hydroxylated silanol]) was free ofany silane(s).

[0040] Water was introduced to the pores of the MCM-41 sample viapassive hydration in a 100% humidity chamber, followed by subjecting thehydrated sample to SCCO₂ forced hydrolysis at 100° C. This hydrationprotocol involved neutral pH, no salt, no ceramic oxide or organiccontaminants; just water, carbon dioxide and heat.

[0041] NMR analysis showed that this hydrolysis treatment was found toincrease the bonding fraction (hydroxylated silicon atoms) to 46% Q3(surface silanol) and 8% Q2 (geminal silanol). This hydrolysis wascarried out in the presence of excess water and hydrolysis stopped atthis point, with no damage to the mesostructure. The mild conditions ofthis hydrolysis prevent the dissolution of the MCM-41 since silicic acidis insoluble in SCCO₂, and thus there was very little risk of collapsingthe mesostructure. (SCCO₂ is very nonpolar, and approximates hexane inits solvating power).

[0042] The hydrolysis reaction was complete in about 20 minutes (SeeFIG. 1).

[0043] A comparison was made to a hydration or hydrolysis done byplacing a second sample of MCM-41 in water and boiling at atmosphericpressure for 4 hours. In this comparison, no change or difference wasobserved.

EXAMPLE 2

[0044] An experiment was conducted to compare the surface density ofassembly molecules using prior liquid solution deposition as reported byFeng et al., 1997 and Liu et al, 1998 (described in Background above),and using the supercritical fluid solution deposition of the presentinvention.

[0045] Surface density was determined gravimetrically and by ²⁹Si NMR.

[0046] The surface density of the product made with the liquid solutiondeposition which included an azeotropic distillation was 5.0-5.2silanes/nm².

[0047] According to the present invention, The silica was hydrated bysimply storing it in a 100% humidity chamber and monitoring the sample'sweight as a function of time, stopping at 20-25% weight gain. The MCM-41was added to the sample holder along with the MPTMS (mercaptopropyltrimethoxysilane), then the system was sealed and brought up to pressureand temperature (7500 psi and 150° C.) with SCCO₂. After only 5 minutes,a monolayer with a surface density of 6.4 silanes/nm² was depositedwhich was surprisingly approximately 20% higher than achieved usingliquid phase deposition.

[0048] The spectrum of this 5-minute sample is shown in FIG. 2a. Areference peak 200 is from TTMS tetrakis (trimethylsilyl) silane. Thepeak 202 is a combination of signals for Q2, Q3, and Q4 silicic acidunits in the base material. The peaks 204, 206, 208 are the internal,terminal, and isolated silanes respectively. The silane demographics ofthis sample are similar to those found in monolayers prepared underatmospheric pressure and liquid solution phase conditions.

[0049] It was also found that maintaining the sample at elevatedtemperature and pressure in SCCO₂ resulted in a slow but steadyevolution of the silane demographics, with a gradual decrease in thepopulation of the terminal silane with a concomitant increase in thepopulation of the internal silane over 24 hours. Over this sametimeframe, the signal for the isolated silane completely disappeared,indicating an annealing of the “dangling” hydroxyls within themonolayer, resulting in a greater fully bonded fraction or higher degreeof siloxane cross-linking. In this experiment, we observed a terminal tointernal silane ratio (based upon total area under each peak) ofapproximately 1:4 after 24 hours (FIG. 2b). This is unexpectedly thehighest degree of crosslinking in a silane based monolayer documented by²⁹Si NMR. Ratios as a function of time are summarized in FIG. 2c showingevolution of silane demographics.

[0050] Both the surprising surface density and the unexpected highdegree of crosslinking are directly attributable to the use of SCCO₂ asthe reaction medium.

EXAMPLE 3

[0051] An experiment was conducted as in Example 2 wherein a portion ofthe material from the 5-minute sample was re-exposed or re-introduced tothe supercritical fluid environment for an additional 30 minutes.

[0052] Results are shown in FIG. 3. The evolution or formation of themonolayer continued in the same manner as for continuous supercriticalfluid exposure.

EXAMPLE 4

[0053] An experiment was conducted to demonstrate the enhanced chemicalstability of the coating material as produced as in Example 2. Samplesof the coating material of the present invention from Example 2 wereexposed to a series of identical buffer solutions of various pH.Comparative samples of solution deposited coating material were exposedto identical buffer solutions of various pH from pH 0.5 to pH 12.5.

[0054] Results are shown in FIG. 4 wherein essentially no difference isobserved for pH less than 9, but above pH 9 up to pH 12.5, the solutiondeposited material (Regular SAMMS) exhibits a leaching or loss of sulfurto the solution indicating a degradation of the monolayer. Thesupercritical deposited material according to the present inventionshows no change in sulfur concentration above pH 9 up to pH 12.5. Thisincrease in chemical durability of the monolayer is an unexpectedresult.

EXAMPLE 5

[0055] An experiment was conducted as in Example 2 wherein Zeolite betafrom (Zeolyst) in the form of beads (3 mm in diameter) was mixed withtris(methoxy)mercaptopropylsilane (TMMPS) with a zeolite to TMMPS weightratio of 0.67 and loaded into the sample container in the supercriticalreaction vessel. The system was sealed. The pressure and the temperaturewere increased to 7500 psi and 150° C. using CO₂. The zeolite wastreated under these conditions for 12 hours before the pressure wasreduced and temperature decreased to 25° C. The treated zeolite beadswere recovered after the treatment, and ground into powders for furthertreatment.

[0056] Functionalized and unfunctionalized zeolites were characterizedby X-ray diffraction (XRD). The XRD peaks recorded are consistent withthe XRD data reported for zeolite beta, but the diffraction peaks arebroader and weaker. Apart from the two main peaks at 2 theta of 7.7° and22.5°, other minor diffraction peaks are not well resolved. The XRD datasuggest that the commercial zeolite beta used here has a similarcrystalline structure as the synthetic high purity zeolite beta, but hassmaller crystallite size, or a higher degree of disordering.Transmission electron microscopy (TEM) images and electron energydispersive X-ray spectroscopy (EDX) spectra were also obtained. Becausethe commercial zeolite beta is not highly crystalline, the zeolitelattice fringes are, not resolved in the TEM image. In the EDX spectrum,a strong Si peak, a small Al peak (from the zeolite material), and asmall S peak is observed. This S peak comes from the sulfonate groupintroduced into the zeolites during SCCO₂ functionalization. From theEDX data the sulfonate group density in zeolite can be estimated to be0.87 mmol sulfonate/g zeolite. The aluminum concentration is about 2% byweight.

[0057] Typically part of the unreacted silanes and by-products would beforced out of the inner volume of the porous substrate when the pressurewas quickly reduced in the supercritical treatment. After thesupercritical treatment, the materials were further treated withH₂O₂/method solutions and H₂SO₄ solutions over long periods of timeduring the acidifying and sulfonation procedure, which also includesmany washing and rinsing steps involving ethanol and water. It isexpected that any physically trapped silanes or its by-products shouldbe removed from the material during these treatments. This conclusion isverified by acid washing experiments. No silane product was released, asmeasured by NMR experiment, when the sulfonated zeolite was subject toextended wash in 0.1 M acid solutions.

[0058] A Chemagnetics NMR spectrometer was used to obtain ²⁹Si NMRresults. It is important to recognize that relative peak intensities in²⁹Si CP-MAS are not strictly quantifiable due to differences inrelaxation behavior. Therefore, we have used the Bloch decay pulsesequence (single pulse excitation) with long recycle times (30 sec) toobtain the spectrum more representative of the molecular composition ofthese materials. The large peak at −110 ppm is from the silica support.The broad feature at −110 ppm is also indicative of the poor crystallinenature of the zeolite. Two additional peaks from −50 to −80 ppmcorresponding to siloxane groups in the functionalized zeolites areobserved. The siloxanes peaks are much more pronounced than reported inliterature, suggesting a higher surface coverage. The peak positionssuggest a high degree of crosslinking between the siloxane groups, andbetween siloxane groups and the substrate. Since the small pore sizeexcludes the possibility of close packing of the silane groups, so mostlikely the siloxanes are attached to the substrate with a tridentate, orbidentate binding. On smooth substrates or in large pore materials, bothtridentate and bidentate bindings are not favored. This binding schemehas been reported in zeolites because of the small pore size and highcurvature.

[0059] The conversion of HEX and PYC can be easily quantified and theonly observed products were the mixtures of reactants and acetalized (orketalized) products. For sulfonated zeolite (Z—SO₃H), more than 60 % HEXwas converted in 4 hours, and nearly complete conversion was observedover 12 hours. On the other hand, PYC, which has a large molecular sizeand cannot enter the microporosity, showed less than 8% conversion overextended reaction time with same Z—SO₃H as catalyst. These resultsindicate that the Z—SO₃H material is size selective, and that themajority of SO₃H groups are inside the microporosity and are accessibleto molecules smaller than the pore size, and inaccessible to themolecules larger than the pore size. The reaction rate with SCCO₂ Z—SO₃Hcompares favorably with similar zeolite functionalized using an in-situdeposition technique, which produced 38% conversion of HEX in 4 hoursunder the same conditions. Both HEX and PYC were also reacted over purezeolite beta (Z), and the TMMPS functionalized zeolite (Z—SH) before itwas treated with H²O². Pure zeolite and Z—SH showed low catalyticactivity, and only a small fraction of either HEX and PYC was converted.It can be concluded from these results that the majority of SO₃H groupsreside inside the zeolite pore channels and act as the active center forthe reaction. It is important to note that no additional extractionprocedure was performed on the supercritically processed zeolite toremove chemically bonded sulfonated groups on the external surface ofthe zeolite. Therefore a small portion of chemically bonded sulfonatedgroups remained on the external surface, which gave rise to the residualactivity observed for PYC. However, the contribution of the externalsulfonic groups to the overall reaction is minimal. Furthermore, theactivity of the external acid groups can be selectively neutralized toachieve complete size selectivity (which has been demonstrated in ourexperiments). The external acid groups can be also removed through apost extraction treatment.

[0060] The high activity of the functionalized zeolite over the parentmaterial is attributed to the acidic groups introduced by the functionalgroups. To verify this, acid-base titration was conducted to determinethe number of acid sites. The titration was conducted in 5.0 ml 0.1 NHCl solution with 0.05 g suspended solid powders using a 0.1 N NaOHsolution as the titrant. The titration curve is the superposition of thetitration curve of the strong acid (HCl) and that of the surface acidgroup from the catalyst. The proton capacity (acid site density) can becalculated using standard methods. The titration experiments showed thatthe sulfonated zeolites (Z—SO₃H) have a much high proton capacity (25.7mmol/g) as compare with the native zeolite beta (3.79 mmol/g) and theunsulfonated thiol zeolite (Z—SH) (3.37 mmol/g).

[0061] For comparison, sulfonated mesoporous silica (M—SO₃H) was used tocatalyze the conversion of PYC and HEX. In this case both HEX and PYCcan easily enter the pore channel and access the catalytic SO₃H sites.Therefore low-selective conversion of both HEX and PYC were observed.

[0062] Further evidence of the size selectivity is provided when aminesof different sizes are used to poison (neutralize) the acid sites.Reaction of HEX and glycol was performed over Z—SO₃H as discussedbefore. Triethylamine ((C₂H₅)₃N, or TEA) is added to the reaction bathafter 40 minutes. TEA is a small molecule and can enter themicroporosity and poison all the acid sites. The addition of TEAcompletely stopped the reaction. Under the same condition the additionof methyldioctylamine [(CH₃(CH₂)₇)₂NCH₃, or MDOA] instead of TEA did nothave any effect on the conversion of HEX over Z—SO₃H, because themolecular size of MDOA is too large for it to enter the microporosityand poison the acid sites in the internal pore channels. The addition ofMDOA did effectively neutralize all the residual acid sites on theexternal surfaces of Z—SO₃H. Zero activity was observed for PYC underthese conditions. If the pore size is large enough, like in mesoporoussilica (M—SO₃H), MDOA is an effective poison for acid catalyzedreaction. We have shown that the addition of MDOA to the reaction bathof HEX over M—SO₃H completely stopped the reaction in the mesoporousmaterials.

CLOSURE

[0063] While a preferred embodiment of the present invention has beenshown and described, it will be apparent to those skilled in the artthat many changes and modifications may be made without departing fromthe invention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

We claim:
 1. A functionalized material having a self-assembled monolayeron a substrate said self-assembled monolayer having a plurality ofassembly molecules each with an assembly atom with a plurality ofbonding sites wherein a portion of fully crosslinked silicon atoms is amaximum portion provided by a liquid solution deposition; wherein theimprovement comprises: said portion of fully bonded silicon atoms isgreater than said maximum portion as provided by supercritical fluidsolution exposure.
 2. The functionalized material as recited in claim 1,wherein said assembly atom is silicon having four bonding sites and saidmaximum portion is 40%.
 3. The functionalized material as recited inclaim 2, wherein said greater portion is greater than 55%.
 4. Thefunctionalized material as recited in claim 3, wherein said greaterportion is greater than or equal to about 75%.
 5. The functionalizedmaterial as recited in claim 1, wherein a surface density of saidplurality of assembly molecules is greater than 5 assembly molecules persquare nanometer.
 6. The functionalized material as recited in claim 5,wherein said surface density is about 6.5 assembly molecules per squarenanometer.
 7. A functionalized material having a self-assembledmonolayer on a substrate said self-assembled monolayer having aplurality of assembly molecules each with an assembly atom with aplurality of bonding sites wherein a surface density of said assemblymolecules is a maximum surface density provided by liquid solutiondeposition; wherein the improvement comprises: said surface density is agreater surface density greater than said maximum surface density asprovided by supercritical fluid solution deposition.
 8. Thefunctionalized material as recited in claim 7, wherein said maximumsurface density is less than or equal to 5 assembly molecules per squarenanometer, and said greater density is greater than 5 assembly moleculesper square nanometer.
 9. The functionalized material as recited in claim7, wherein said plurality of assembly molecules has a greater portion offully bonded assembly atoms greater than a portion of fully bondedassembly atoms.
 10. The functionalized material as recited in claim 9,wherein said greater portion of fully bonded silicon atoms is greaterthan 55%.
 11. The functionalized material as recited in claim 10,wherein said greater portion of fully bonded silicon atoms is greaterthan or equal to about 75%.
 12. The functionalized material as recitedin claim 7, wherein said substrate is a mesoporous material.
 13. Amethod of making a self-assembled monolayer on a substrate saidself-assembled monolayer having a plurality of assembly molecules eachwith an assembly atom with a plurality of bonding sites, the methodhaving the step of bonding a plurality of oxygen atoms to a fraction ofsaid bonding sites; wherein the improvement comprises: said bonding isby exposing said self-assembled monolayer to supercritical fluid. 14.The method as recited in claim 13, wherein said supercritical fluid iscarbon dioxide.
 15. The method as recited in claim 13, wherein saidexposing is for a time of at least 5 minutes.
 16. The method as recitedin claim 15, wherein said exposing is for a time of at least 4 hours.17. The method as recited in claim 16, wherein said exposing is for atime of about 24 hours.
 18. The method as recited in claim 13, whereinsaid exposing comprises: placing a self-assembled monolayer precursortogether with a calcined mesoporous material into a vessel; followed byintroducing a supercritical fluid into said vessel for a time anddepositing said self-assembled monolayer onto said calcined mesoporousmaterial.
 19. The method as recited in claim 18, wherein said placingsaid calcined mesoporous material comprises: mixing a sol-gel solutionand surfactant for producing a mesoporous green body; removing saidsurfactant with said supercritical fluid and making a dry green body;calcining said dry green body into said calcined mesoporous material.20. A functionalized porous material comprising a substrate, a monolayerdeposited on said substrate having a plurality of assembly moleculeseach with a plurality of assembly atoms having a plurality of bondingsites; and said assembly atoms being greater than 70% fully cross linkedwith cross linking atoms.
 21. The material of claim 20, wherein thesubstrate is a porous ceramic material.
 22. The material of claim 21,wherein a pore size of said ceramic porous material is greater than 5 Å(angstroms).
 23. The material of claim 20, wherein said assembly atom issilicon.
 24. The material of claim 20, wherein said assembly moleculesare selected from the group consisting of metal phosphate, hydroxamicacid, carboxylate, thiol, amine and combinations thereof for attachingto a metal.
 25. The material of claim 20, wherein said assemblymolecules comprises chlorosilane for attaching to a polymer.
 26. Thematerial of claim 20, wherein the monolayer comprises a surface densityof assembly atoms greater than 6.4 silanes/nm².
 27. The material ofclaim 20, wherein the cross linking atom is oxygen.
 28. A method offunctionalizing a porous material comprising the steps of a) hydrating aporous substrate; b) placing said hydrated porous substrate into avessel in the presence of a precursor comprising assembly atoms andcross linking atoms; c) sealing the sample holder; d) introducing asupercritical fluid into said vessel for a time while increasing thepressure and temperature within said vessel to a supercritical pressureand temperature; e) maintaining the porous substrate within said vesselunder supercritical conditions for a time sufficient to fully cross linkat least 70% of said assembly atoms with said cross linking atoms on theexposed surface of said materials.
 29. The method of claim 28, wherebysaid porous substrate is ceramic.
 30. The method of claim 29, wherebysaid ceramic porous substrate comprises pores having a diameter greaterthan 5 Å.
 31. The method of claim 28, whereby hydrating said poroussubstrate is provided by a humidity chamber having a humidity of 100%.32. The method of claim 28, whereby said assembly atom is silicon. 33.The method of claim 28, whereby said supercritical fluid is carbondioxide (CO₂), ethane (C₂H₆), ammonia (NH₃), and combinations thereof.